A polyvinyl acetal represented by polyvinyl butyral has excellent adhesion and compatibility with various organic and inorganic substrates, and has excellent solubility in an organic solvent. The polyvinyl acetal is widely used as various adhesives, a binder for ceramics, various inks, paints, and the like, or an interlayer film for safety glass.
Recently, in an application of an interlayer film for laminated glass, various highly functional products have been developed. For example, a laminated sound insulation interlayer film for laminated glass is disclosed (for example, refer to Patent Literatures 1 and 2). In the laminated sound insulation interlayer film for laminated glass, in order to impart high sound insulating properties to the interlayer film for laminated glass, a plurality of polyvinyl acetal layers having different compositions is laminated. For example, the polyvinyl acetal layers have different content ratios of a polyvinyl acetal and a plasticizer. In the interlayer film for laminated glass, in general, in order to make amounts of a plasticizer included in the respective layers different from each other, polyvinyl acetals having different average amounts of remaining hydroxyl groups between the respective layers are used.
In the interlayer film for laminated glass made from a composition including a polyvinyl acetal and a plasticizer, in addition to mechanical strength and adhesion to glass, required for an interlayer film for laminated glass, water resistance such as low water absorbency is required. Triethylene glycol di(2-ethylhexanoate) has low polarity and a high boiling point. A composition obtained by mixing triethylene glycol di(2-ethylhexanoate) with a polyvinyl acetal, and a sheet made from the composition exhibit, in a well-balanced manner, properties required for an interlayer film for laminated glass, such as mechanical strength, adhesion to glass, and water resistance. Therefore, triethylene glycol di(2-ethylhexanoate) is used particularly often as a plasticizer in this field (for example, refer to Patent Literature 3).
By the way, in general, an interlayer film for laminated glass is manufactured by extrusion molding from a viewpoint of production costs thereof. The laminated sound insulation interlayer film for laminated glass is manufactured by a coextrusion method. However, when the interlayer film for laminated glass is manufactured by this method, a trim or an off-spec product which cannot be a commercial product because of nonuniform composition or thickness is generated in a certain amount.
In general, such a trim or an off-spec product can be recycled by melt kneading and extrusion molding again. However, when a mixture of polyvinyl acetal compositions having different compositions as a trim or an off-spec product is used, a resulting interlayer film for laminated glass may be opaque. In particular, in the laminated sound insulation interlayer film for laminated glass, the polyvinyl acetal compositions included in the respective layers include polyvinyl acetals having different average amounts of remaining hydroxyl groups from each other. Therefore, it is difficult to make the polyvinyl acetals compatible with each other. The interlayer film for laminated glass obtained by recycling the trim or the off-spec product generated during manufacturing the laminated sound insulation interlayer film for laminated glass has a problem of poor transparency. When the polyvinyl acetal composition includes triethylene glycol di(2-ethylhexanoate) as a plasticizer, there is a significant problem that the interlayer film for laminated glass obtained by recycling the trim or the off-spec product has poor transparency.